It is ages since an apparently carefree Jamie Oliver and his band were Dancing in the Moonlight. Nowadays, he is singing to a banker's beat. He might wear jeans hanging off his bottom, but in a deeper sense the Naked Chef is clad in City chalk stripes.

Television shows, cookery books and campaigns have made him an ethically-responsible multinational corporation. Even his scooter - emblem of carefree, youthful loft-living with gorgeous friends swinging by for bacon sarnies - has become a lucrative logo, as identifiable as a McDonald's twin arch.

Yet Oliver has made himself a conglomerate while remaining a national treasure. With the exception of David Beckham, he is the only public figure who could saunter down a street propositioning women randomly without receiving a slap. At worst, he would be scolded in an indulgent "Ooh, you are awful" way. It is not even because he is attractive, though the happily married father surely is; it is that people across the ages and classes warm to him massively. Once it was deemed bad form to gyp royalty; now in self-consciously hip circles it is virtually de trop not to. Only the pukka personage of our Jamie remains above criticism: what's not to revere about this knight of the garlic?

Until now the area that Brand Jamie had not exploited was restaurants. He set up Fifteen for charity and The Cock for television, but with the opening of his latest venture in Oxford he is moving into commercial restaurants big time.

Jamie's Italian is to be a chain. Boo, hiss. At least Gordon Ramsay acts as if he wants to individualise restaurants; not Oliver. Jamie, in keeping with his cheap-as-chips persona, aspires not to "exclusivity" but inclusiveness. This human face of a supermarket wants to pile it high, sell it cheap - but nicely: good food costs less when it's Jamie.

The model is Carluccio's with its buzzy mood and affordable Italian tucker, just without the controversy over what its waiters are paid. Restaurant staff can often effectively be paid below the minimum wage, the view being that tips will make up the difference (or, failing that, the company concerned). But in my view tips should be extra, not a wage.

Anyway, Oliver's first branch in Oxford will be followed by Bath and Kingston and then, presumably, the world. It's a shrewd move. There are swanky restaurants in the surrounding countryside, but when it comes to gastronomy, Oxford city centre is an empty larder. The Randolph, where parents have long toasted Firsts and commiserated with Thirds, is so bad that diners may feel they are being set up in a remake of Candid Camera. Jamie's arrival here is seen as the greatest culinary thrill since Raymond Blanc's Le Manoir aux Quat' Saisons three decades ago.

You spot the restaurant by the queue. You can't book and, despite it being open for breakfast, lunch and dinner, everyone from cronky don to cool student-cum-extra from an Oliver ad for Sainsbury's lines up patiently. But when you finally gain admittance you'll find the front of house chatting to customers rather than seating them. The downstairs - decorated, a little self-consciously, in graffiti - is empty. If the queue is artificially induced, it still creates an electric atmosphere. Entire hams hang from the ceiling and a strategically placed pasta machine hints at the rustic Tuscan cuisine, yet the decor is urban English. A confusing jumble? Locals seem to find it lovely jubbly.

We are shown to a communal table and eventually offered excellent prosecco-based cocktails. At £4.25 a shot, they are knocked back with gusto, fuelling the lunchtime party feel. At the bar, hungry diners awaiting tables watch TV monitors that show food being prepared. The menu - which smartly mixes cheapish antipasti and pasta (to draw students) with pricier grills - even treats us to feed-the-world Jamie rhapsodising, about how Italians are besotted with food, whether "rich or poor". This noble eulogy is only slightly tarnished by an advert underneath for myriad Jamie products, including £15 T-shirts. Oliver's presence is everywhere, really, except the kitchen. Gennaro Contaldo, his friend and mentor, is briefly in charge. He will then move on to open the next "Jamie's" in the chain.

I follow with "good old-fashioned grill steak" (£15.95), described as "pink". I'm also asked how I'd like it. Answer: red. It arrives brown and rubbery. I give up and focus on delicious polenta chips scattered with rosemary and sea salt (£2.50). Diana's chargrilled king prawns (£16.50) are attractively clothed in herbs on a wooden board, but so dripping in dressing that we are soon swimming in an oil slick. They taste fine, though. Puddings, like tiramisu, are predictable but good.

There is so much to improve, even with Contaldo still in charge. Yet this will still be the best mid-market restaurant in town, wherever Oliver opens. Where, for instance, can you find really good fresh pasta on the high street? Driving up food standards in provincial Britain is, as Oliver might say, an "awesome" challenge, equal to improving school grub. But please, Jamie, while you are dancing your victory jig in the boardroom, don't leave your fans outside - singing, not in the moonlight, but in the rain.